Cold cathode gas discharge tubes having an optically opaque masking plate



3,37%,195 Patented Feb. 20, 1968 3,370,195 COLD CATHODE GAS DISCHARGETUBES HAVING AN OPTICALLY OPAQUE MASK- ING PLATE George Peter Stenning,Stanrnore, England, assignor to Hivac Limited, South Ruislip, Middlesex,England, a British company Filed Get. 12, 1965, Ser. No. 495,239 Claimspriority, application Great Britain, Oct. 16, 1964, 42,349/64 4 Claims.(Cl. 313-204) ABSTRACT 01? THE DISCLOSURE Cold cathode gas dischargeindicator tube having several shaped cathode electrodes in front of atleast one of which is an optically opaque masking plate provided with alight-transmitting aperture therein to permit visual observation of onlythose parts of the glow discharge on such cathode electrode whichprovide the required shape of visual indication.

This invention relates to cold cathode gas discharge tubes and is moreparticularly concerned with devices, commonly referred to as indicatortubes, in which a plurality of separate cathode electrodes, in stackedrelationship, are provided with individual external electricalconnections thereto for selective cooperation with a common anode eachcathode electrode being of different shape and so arranged as to providea variety of visual glow discharge displays having the respective formsof different characters such as letters, numerals, algebraic signs, andthe like.

In the construction of such tubes it is necessary to support each of thealternative character electrodes in a separately insulated manner andthis is usually effected by providing each electrode with two or moreoutwardly directed extensions or lugs which serve as anchorage pointsfor mounting each of the characters of the stack upon spaced apartsupport rods disposed normal to the respective planes of the differentelectrodes and located just outside the boundary of the viewing area inwhich each of the different discharge displays occurs. Such support rodsare usually located at opposite sides of the stack, e.g. one above andthe other below in the vertical median plane of the stack when theelectrode planes are vertical.

While such disposition is the most convenient for most numerals and manyalphabetical characters, such as B, C, D, E, G, I, O, Q, S, T, and Zowing to the existence of regions of the required character shapeimmediately adjacent the anchorage points, certain other characters suchas H, J, K, L, M, N, P, R, U, V, W, X and Y present difficulties andneed the use of one or more extensions to connect the character-shapedportion of the electrode to one or both of the anchorage points. It isclearly most important that such extensions should not glow and specialmeasures need to be taken to prevent any glow discharge thereon such asby covering them with a layer of insulation. A particularly difficultelectrode shaping to provide is one in which the required displayisconcentrated in the mid region between the various support rods, forexample, an algebraic minus sign and one object of the invention is theprovision of an improved form of indicator tube construction fordisplaying such character shapings.

In accordance with one aspect of this invention a cold cathode gasdischarge tube comprises an anode electrode, a cathode electrodearranged to provide a visual glow display and comprising a firstcomponent part shaped in accordance with the required display and one ormore further parts extending therefrom to fixed anchorage points, and anopaque masking plate disposed in front of said cathode electrode andhaving an aperture therein shaped to allow visual observation of theglow discharge on said first component part when viewed in the normaltube viewing direction.

In certain applications of such indicator tubes, such as in connectionwith electronic computers, it is frequently necessary or is verydesirable that a substantially constant value of glow discharge currentflow be maintained in an associated circuit regardless of whether or notany one of the character-shaped cathode electrodes is being activelyused to provide a visual indication. Hitherto in such case it has beenthe practice to provide a separate glow discharge tube having broadlysimilar current and voltage characteristics and arranged in analternative circuit path which is switched into operation whenever theindicator tube itself is not required to provide a visual indication.Such arrangement has certain disadvantages, firstly, the need to providea second discharge tube and, secondly, the need, at least approximately,to match the characteristics of the second tube to those of theindicator tube. Even with such initial matching balanced operatingconditions will probably not be maintained over a prolonged period oftime owing to differences of tube construction, and the differentintegrated usage of the two tubes.

In accordance with a second aspect of the invention a cold cathode gasdischarge tube comprises an anode electrode, at least one displaycathode electrode arranged to provide a visual glow display and anadditional cathode electrode the glow from which is totally obscured byan opaque masking plate, the area of said additional cathode over whichdischarge takes place being substantially equal to the correspondingarea of said display cathode or to the average of the correspondingareas of said display cathodes.

In order that the above and other features of the invention may be morereadily understood one particular embodiment thereof will now be brieflydescribed with reference to the accompanying drawing in which:

FIGURE 1 is a perspective View of a gas discharge tube according to oneembodiment of the invention; and

FIGURE 2 is a vertical section of part of the electrode structure of thetube shown in FIGURE 1 taken along a line just inside the anodeenclosure, part of the anode electrode being omitted for the sake ofclarity.

Referring to the drawings, 10 indicates a tubular glass envelopecontaining a suitable low pressure gas filling. Within this envelope isarranged a first character cathode electrode 11 in the form of analgebraic plus sign constituted by a cruciform thin wire elementanchored at the upper and lower extremities of its vertical limb 12 tohorizontal fixed supports 13, 14 forming part of a support structure(not otherwise shown) extending from the lower end of the tube. It willbe understood that, in the usual way, suitable insulation, e.g. in theform of ceramic bushes 15, is provided at each anchorage point.Immediately behind the said first cathode electrode is disposed avertical sheet metal mask plate 16 having a horizontal slot 17 therein.Immediately behind this mask plate 16 is disposed a further cathodeelectrode 18 again of cruciform shape similar and preferably identicalwith that of the aforesaid electrode 11. This second electrode isdisposed with its horizontal portion 19 in alignment with the slot 17when the tube is viewed in the normal viewing direction indicated by thearrow a. Behind the second cathode electrode 18 is disposed another maskplate 20 which is wholly opaque and behind this is disposed a thirdcathode electrode 21. This third cathode electrode should have the samesurface area as each of the other cathode electrodes and may, forconvenience of manufacture, be identical in form with that of the othertwo. This third electrode 21 is likewise anchored at its upper and lowerends to the horizontal supports 13, 14 by way of insulating bushes orthe like. The assembly is surrounded on three sides at least by abox-like anode structure 22 and the front, open end of the latter mayitself be covered by a fine wire mesh 23 (not shown in FIGURE 2) whichpermits viewing of any glow discharge on the first or second electrodestherethrough. The mask plates 16, 20 are preferably also connectedelectrically to the anode structure and form part thereof. Leads 24, 25,26 are connected respectively to the separate cathode electrodes 11, 18and 21 respectively while lead 27 is connected to the common anode.

In operation when the first electrode 11 is energised as a cathode forcooperation with the anode, the full cruciform or plus sign shape ofvisible display will be provided. When the second cathode electrode 18is energised instead, although a cruciform glow discharge will againoccur, only the horizontal limb 19 thereof will be visible through theslot .17 in the mask plate 16 thereby providing a minus sign indication.Preferably such horizontal limb and the aligned slot 17 in the maskplate are slightly displaced upwardly or downwardly with respect to thehorizontal limb of the front electrode 11 to prevent any obscuring bythe latter of the glow discharge when the second electrode 18 isenergised. To facilitate manufacture using identical electrode shapesthe horizontal limb may be placed slightly off-centre of the verticallimb and one electrode inverted relative to the other.

With the third electrode 21 energised, while a further flow discharge ofcruciform shape will again occur, none of this will be visible and thetube is, outwardly, in the state providing no visualindication at all.Such third discharge, however, will have substantially the same Ielectrical characteristics as either of those of the first two andaccordingly provides a discharge which is substantially balanced asregards its voltage and current requirements with those of the other twodischarges. Furthermore, as the third electrode is itself located in thesame gas discharge space its characteristics will tend to altercorrespondingly with those of the first two and the balanced conditionwill be maintained throughout the life of the tube.

While the invention has been described in detail with relation to theprovision of and signs its manner of application to other characterforms will be obvious.

I claim:

1. A cold cathode gas discharge tube comprising an anode electrode, atleast one display cathode electrode arranged to provide a visual glowdisplay and an additional cathode electrode the glow from which istotally obscured by an opaque masking plate, the area of said additionalcathode over which discharge takes place being substantially equal tothe corresponding area of said display cathode or to the average of thecorresponding areas of said display cathodes.

2. A gas discharge tube as claimed in claim 1 in which the anodeelectrode is in the form of a box surrounding the cathode electrodes, atleast part thereof being formed from wire gauze or the like.

3. A cold cathode gas discharge tube comprising an anode electrode, apair of cathode electrodes spaced and insulated from each other, saidcathode electrodes having substantially the same size and shape, anoptically opaque masking plate disposed between said cathode electrodesand having a light-transmitting aperture therein shaped and positionedto allow visual observation of only a portion of the glow discharge ofone of said cathode electrodes whereby the cathode electrodes displaydifferent characters when the tube is viewed in the normal direction ofobservation.

4. A gas discharge tube as claimed in claim 3, said pair of cathodeelectrodes being cruciform, said aperture being in the form of ahorizontal slot that exposes to view only the cross arm of one of thecathode electrodes where by the cathode electrodes display algebraicplus and minus signs.

References Cited UNITED STATES PATENTS 2,796,558 6/1957 Koehler3l3--l09.5 3,244,923 4/1966 Milliken 313109.5 3,248,589 4/1966 Despris313-1095 JAMES W. LAWRENCE, Primary Examiner.

R. L. JUDD, Assistant Examiner.

